This invention relates to a method of fabricating a magnetoelastic torque sensor. More particularly, this invention relates to fabrication of a magnetoelastic torque sensor having a transducer ring affixed to a substrate in a plating process.
Conventional non-contact torque sensors include a magnetoelastic ring that is supported on a shaft. A uniaxial, circumferential stress anisotropy is created in the magnetoelastic ring such that a circumferential magnetic anisotropy results, creating a predisposition within the transducer element toward a circumferential magnetic field within the magnetoelastic ring. The circumferential magnetic field becomes distorted responsive to surface shear stress resulting from an applied torque to the shaft. The amount and direction of magnetic field distortion is measured and provides a value utilized to provide the desired torque measurements.
Conventional methods of fabricating a torque sensor include the pressing of a ring of positively-magnetostrictive material onto a magnetically inert shaft. The shaft includes a taper such that a press fit of the ring onto the shaft will produce the desired circumferential tension or “hoop stress”. The circumferential tension creates a magnetic easy axis and thereby facilitates the stability of the circumferential magnetic field. The press fit between the ring and the shaft is a limiting factor to the capability and accuracy of such torque sensors. Any slipping or relative movement between the shaft and the ring distorts the actual reading of torque and causes a shift in the zero point of any torque measurement.
A known improvement over press fitting of rings onto a shaft includes the use techniques such as thermal-spraying or kinetic metallization to apply a magnetoelastic material, particularly nickel, onto a non-ferromagnetic stainless steel substrate. The desired stress anisotropy for the magnetoelastic material is provided by applying an axial load and heat to the substrate during the application of the magnetoelastic material. Once applied, the substrate is cooled and the axial load released. This results in an axial tensile stress and a compressive circumferential hoop stress on the magnetoelastic materials, thereby producing the desired magnetic field. However, such a fabrication method is difficult to control, requires the use of an expensive grade of stainless steel for the substrate, and wastes much of the magnetoelastic material during the thermal spraying process.
Accordingly, it is desirable to develop a method of fabricating a magnetoelastic torque element that utilizes less expensive materials in a more reliable manner.